A photo-degradable helix: Synthesis, structure, and photolysis of optically active poly[2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate]

Article abstract of DOI:10.1002/pola.24507

2,7-Bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate (BBPMFA) was synthesized and polymerized using α,α′-azobisisobutyronitrile or n-Bu₃B-air as a radical initiator and using the complex of 9-fluorenyllithium with (S)-(+)-1-(2-pyrrolidinylmet

Full text of DOI:10.1002/pola.24507

A Photo-Degradable Helix: Synthesis, Structure, and Photolysis of
Optically Active Poly[2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl Acrylate]
TAKESHI SAKAMOTO,1 KENTO WATANABE,1 YUKATSU SHICHIBU,2 KATSUAKI KONISHI,2
SHIN-ICHIRO SATO,1 TAMAKI NAKANO1
1Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, Hokkaido University,
Sapporo 060-8628, Japan
2Faculty of Environmental Earth Sciences, Hokkaido University, Sapporo 060-0810, Japan
Received 21 September 2010; accepted 20 November 2010
DOI: 10.1002/pola.24507
Published online 17 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: 2,7-Bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate
(BBPMFA) was synthesized and polymerized using a,a0-azobisi-
sobutyronitrile or n-Bu3B-air as a radical initiator and using the
complex of 9-fluorenyllithium with (S)-(þ)-1-(2-pyrrolidinylme-
thyl)pyrrolidine as an optically active anionic initiator. Although
the radical polymerization led to rather low-molecular-weight
products at low yields, the anionic polymerization afforded
polymers with higher molecular weights in higher yields. The
poly(BBPMFA) obtained by the anionic polymerization was
slightly rich in isotacticity (meso diad 57%) and showed an
intense circular dichroism (CD) spectrum and large dextrorota-
tion. The intensity of the CD spectrum and magnitude of opti-
cal activity increased with an increase in Mn, suggesting that
the polymer possesses a preferred-handed helical conforma-
tion. The CD spectrum disappeared within 1 s on irradiation to
the polymer in a CHCl3 solution using a 500-W Hg-Xe lamp.
This was ascribed to fast photolysis of the ester linkage leading
to a loss of helical conformation of the entire chain. Photolysis
products of poly(BBPMFA) were poly(acrylic acid) and 2,7-
bis(4-t-butylphenyl)-9-methylenefluorene (2,7-bis(4-t-butylphe-
nyl)dibenzofulvene). The photolysis reaction seemed to pro-
ceed through the ‘‘unzipping’’ mechanism. The rate constant of
photolysis of poly(BBPMFA) under irradiation at monochro-
mated 325 nm was around 0.01 sꢀ1 independent of molecular
weight. Photolysis at 325 nm was approximately 2400 times
faster than that for chemical ester solvolysis under a neutral
C
V
condition in the dark.
2010 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 945–956, 2011
KEYWORDS: anionic polymerization; chiral; photochemistry
INTRODUCTION Chiral polymers are important not only
from the view of basic polymer stereochemistry but also
from a practical view because such polymers have high
potentialities in applications such as chiral separation, non-
linear optical devices, and chiral catalysts.1 One of the most
extensively and systematically studied classes of chiral poly-
mers is a helical polymer. Optically active, helical polymers
with single-handed or preferred-handed helicity can be typi-
cally prepared by asymmetric polymerization of a properly
designed achiral or chiral monomer (helix-sense-selective
polymerization) as well as the other methods including
supramolecular interaction control between an achiral poly-
mer and chiral added molecules,1(j–m) or by chain folding
control of ‘‘foldamers.’’1(g,h) Helix-sense-selective polymeriza-
tion of vinyl monomer was first realized for bulky triphenyl-
methyl methacrylate,2–4 and thereafter, many bulky acrylic
monomers were designed and subjected for this type of
polymerization.1(a–f),5
eomutation on heating in a solution while less bulky esters
including diphenyl-2-pyridylmethyl ester6 undergo helix-helix
transition leading to a remarkable change in chiroptical
properties. As the first example of a helical polymer that
undergoes photo-induced helix-helix transition without any
changes in chemical bonding such as isomerization of a dou-
ble bond or bond cleavage and formation, we recently syn-
thesized poly[2,7-bis(4-t-butylphenyl)-9-fluoren-9-yl acrylate]
[poly(BBPFA)]7 (Chart 1). The conformation of this polymer
is stable on heating in a solution but the polymer readily
undergoes helix-helix transition upon photo irradiation lead-
ing to complete racemization of preferred-handed helix con-
structed through the polymerization process. It was pro-
posed that the helix-helix transition of the chain is triggered
by twist-to-coplanar, local conformational transition of the
side-chain biphenyl moieties of poly(BBPFA) on photo
excitation.
Stimulated by the behavior of poly(BBPFA), in this work, we
synthesized and polymerized 2,7-bis(4-t-butylphenyl)-9-
methylfluoren-9-yl acrylate (BBPMFA) with an additional
methyl group at the 9-position of fluorene ring of BBPFA
The stability of single-handed helix of acrylic polymers
depends on the structure of side-chain group. As for poly-
methacrylates, triphenylmethyl ester does not undergo ster-
Correspondence to: T. Nakano (E-mail: nakanot@eng.hokudai.ac.jp)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 945–956 (2011) 2010 Wiley Periodicals, Inc.
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
945

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
adding saturated aq. NH4Cl (300 mL). The product was
extracted with CHCl3, and the organic layer was dried on
MgSO4. Concentration of the dried organic layer afforded a
crude oily material. The crude material was recrystallized
from a mixture of CHCl3 and MeOH (1/1, v/v) to give a col-
ꢁ
orless crystalline material (39.5 g, 95.0%). Anal: m.p. 266 C
(decomp.); 1H NMR (400 MHz, CDCl3, Me4Si) d 1.36 (s, 18H,
ABut), 2.04 (s, 1H, -OH), 1.80 (s, 3H, ACH3 attached to the
fluorene ring), 7.46–7.47 (m, 4H, aromatic H), 7.58–7.61 (m,
6H, aromatic H), 7.66–7.68 (d, 2H, aromatic H), 7.79–7.80 (d,
2H, aromatic H); HRMS (EI) Calcd for C34H36O1 460.27661,
found 460.27595.
CHART 1 Structures of poly(BBPFA) and poly(BBPMFA).
and investigated the resulting polymer’s structural transition
through the interaction with light. Although our original
intention was to tune the rate of photo-induced helix-helix
transition by steric effects of the additional methyl group,
helical poly(BBPMFA) unexpectedly underwent very fast
photolysis. The photolysis took place through the cleavage of
side-chain ester bonding and the products consisted mainly
of 2,7-bis(4-t-butylphenyl)-9-methylenefluorene and poly
(acrylic acid). The reaction completed within 1 s on irradia-
tion using a 500-W Hg-Xe lamp. This may be one of the fast-
est example among macromolecular reactions so far
reported. Here, we report the full details of synthesis and
polymerization of BBPMFA and photolysis of helical
poly(BBPMFA).
Synthesis of BBPMFA
In a 1-L flask equipped with a dropping funnel and a reflux
condenser flushed with N2 gas was placed 2,7-bis(4-t-butyl-
phenyl)-9-methylfluoren-9-ol (30.0 g, 65.2 mmol). CH2Cl2
(200 mL) and Et3N (13.2 g, 130 mmol) were introduced to
dissolve the alcohol. Acryloyl chloride (10.0 g, 111 mmol)
was slowly added to the solution of alcohol cooled at 0 ꢁC.
The mixture was warmed to room temperature and stirred
for 30 min, and it was then heated to 40 ꢁC and stirred for
1.5 h. The reaction mixture was decomposed by adding
water. The product was extracted with CHCl3-water. The or-
ganic layer was washed with saturated aq. Na2CO3 and with
brine in this order and was then dried on MgSO4. Removal
of solvents gave a crude material. This material was recrys-
tallized from a mixture of toluene and MeOH (10/1, v/v) to
afford a colorless crystalline product (18.1 g, 54.0%). Anal:
m.p. 235 ꢁC; 1H NMR (270 MHz, CDCl3, Me4Si) d 1.35 (s,
18H, ABut), 1.91 (s, 3H, ACH3 attached to the fluorene ring),
1.99 (s, 3H, ACH3), 5.70–5.74 (dd, J ¼ 1.7 Hz, 10.3 Hz, 1H,
vinyl H), 6.00–6.11 (dd, J ¼ 10.3 Hz, 1H, 16.8 Hz, vinyl H),
6.21–6.28 (dd, 1.7 Hz, 16.8 Hz, 1H, vinyl H), 7.44–7.47 (m,
4H, aromatic H), 7.55–7.62 (m, 6H, aromatic H), 7.70–7.73
EXPERIMENTAL
Materials
Acryloyl chloride (Wako Chemical) was purified by distilla-
tion under N2 atmosphere. Fluorene (Nacalai Tesque) was
first recrystallized from ethanol and then from hexane; m.p.
104.5–105.0 ꢁC. The chiral ligand, (S)-(þ)-1-(2-pyrrolidinyl-
methyl)pyrrolidine (PMP) (Aldrich), was dried over CaH2
and distilled under reduced pressure. a,a0-Azobis(isobutyro-
nitrile) (AIBN) (Wako) was recrystallized from an ethanol so-
lution at room temperature. n-Bu3B (1.0 M, a tetrahydrofu-
ran (THF) solution, Aldrich) was used as obtained. MeMgBr
(3.0 M, a Et2O solution, Aldrich) was used as obtained. n-
BuLi (1.6 M, a hexane solution, Kanto Chemical) was used af-
ter titration. Dithranol (Aldrich) and silver trifluoroacetate
(Aldrich) were used as obtained. THF (Wako) and benzene
(Wako) were refluxed over sodium benzophenone ketyl and
distilled under N2 atmosphere. CH2Cl2 was refluxed over
P2O5 and distilled under N2 atmosphere. Et3N was refluxed
over CaH2 and distilled under N2 atmosphere. Toluene used
for anionic polymerization was purified in the usual manner,
mixed with a small amount of n-BuLi, and distilled under
high vacuum immediately before use.
(m, 4H, aromatic H); HRMS (EI) Calcd for
514.28718, found 514.28541.
C37H38O2
Synthesis of 2,7-bis(4-t-butylphenyl)-
9-methylenefluorene (2,7-bis(4-t-
butylphenyl)dibenzofulvene)
In a 1-L flask equipped with a dropping funnel, 2,7-bis(4-t-
butylphenyl)-9-methylfluoren-9-ol (10.0 g, 21.7 mmol) was
dissolved in benzene (500 mL) and the solution was heated
at 80 ꢁC. p-TsOH-H2O (2.10 g, 10.9 mmol) was added to the
solution and the mixture was stirred under refluxing for 20
min. The reaction was decomposed by adding saturated aq.
NaHCO3 (300 mL). The product was extracted twice with
benzene and the combined organic layer was washed with
brine and was then dried on MgSO4. Removal of solvents
gave a crude material (9.64 g, 99.4%). Recrystallization of
the crude material from a mixture of benzene and hexane
(10/1, v/v) afforded a pure crystalline product (6.40 g,
Synthesis of 2,7-bis(4-t-butylphenyl)-9-
methylfluoren-9-ol
The reactions were performed under N2 atmosphere. 2,7-
Bis(4-t-butylphenyl)-9-methylfluorenone was available from
our recent work.7 This ketone (40.0 g, 90.6 mmol) was dis-
solved in THF (300 mL) in a 500-mL flask equipped with a
dropping funnel. To this solution was added MeMgBr (60.0
mL, 180 mmol), and the reaction mixture was stirred at
room temperature for 3 h. The reaction was quenched by
1
ꢁ
66.0%). Anal: m.p. 256 C; H NMR (270 MHz, CDCl3, Me4Si)
d 1.37 (s, 18H, ABut), 6.16 (s, 1H, vinyl H), 7.47–7.50 (m,
4H, aromatic H), 7.58–7.62 (m, 6H, aromatic H), 7.72–7.75
(d, J ¼ 7.8 Hz, 2H, aromatic H), 7.93–7.94 (d, J ¼ 2.2 Hz, 2H,
aromatic H); HRMS (EI) Calcd for C34H34 442.26605, found
442.26482.
946
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
TABLE 1 Polymerization of BBPMFA Using Various Initiatorsa
MeOH-Insoluble Part
Temp
(ꢁC)
Conv.b
(%)
Yield
(%)
[a]435
(deg)
[a]D
d
d
½Mꢃ
c
c
Run
Initiator
Solvent
Mn
Mw/Mn
(deg)
½Iꢃ
60
30
5
1
2
3
4
5
6
7
8
9
a
AIBN
CHCl3
60
ꢀ30
ꢀ78
ꢀ78
ꢀ78
0
20
8
15
5
1200
700
1.40
1.10
1.12
1.12
1.18
1.36
2.26
3.04
1.41
–
–
n-Bu3B-aire
PMP-FILi
PMP-FILi
PMP-FILi
PMP-FILi
PMP-FILi
PMP-FILi
PMP-FILi
THF
–
–
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
74
69
42
>99
>99
62
58
d
62
60
34
91
90
52
55
3100
2800
2300
2800
5400
10,200
1300f
þ180
þ142
þ87
þ116
þ282
þ279
–
þ47
þ35
þ23
þ26
þ78
þ75
–
10
20
5
10
15
20
0
0
0
Conditions: BBPMFA, 0.100 g (run 1), 0.500 g (run 2), and 0.515 g (run
3–9); [BBPMFA]0 ¼ 1.00 M (run 1), 0.65 M (run 2), and 0.080 M (run 3–
9); [FILi]/[PMP] ¼ 1.2; polymerization time 12 h (run 1), 6 h (run 2), 24 h
(runs 3–9).
In CHCl3 at room temperature (conc. ¼ 0.50 g/dL; cell length ¼ 1 cm).
Air (5 mL) was bubbled through the monomer solution containing n-
e
Bu3B.
f
THF-soluble part only.
b
Determined by 1H NMR analysis of reaction mixture.
c
Determined by SEC in THF (vs. PSt) (run 1–8) and, in CHCl3 (vs. PSt)
(run 9).
Polymerization
mL glass tube with a screw cap in the dark. After the solu-
tion was heated at 60 ꢁC for 9 h (conversion 90% by 1H
NMR analysis), 1 mL of MeOH was added to the solution.
Radical polymerization using n-Bu3B was carried out by the
literature method.8 Preparation of initiator solution and
asymmetric anionic polymerization were carried out in dry
toluene under dry N2 atmosphere. Polymerization procedure
is described for run 6 in Table 1 as an example. Fluorene
(0.0898 g, 0.540 mmol) was placed in a flame-dried glass
ampoule sealed with a three-way stop cock, which was then
evacuated on a vacuum line and flushed with dry N2 gas. Af-
ter this procedure was repeated three times, a three-way
stop cock was attached to the ampoule and dry toluene
(5.10 mL) was introduced with a syringe to dissolve fluo-
rene. n-BuLi in hexane (0.340 mL, 0.540 mmol) and (þ)-
PMP (0.100 g, 0.650 mmol) were added to the resultant so-
lution with a syringe in this order, and the mixture was left
to stand at room temperature for 10 min to obtain an or-
ange-colored PMP-FlLi complex solution (0.100 M). In a simi-
lar manner to the preparation of the fluorene solution,
BBPMFA (0.515 g, 1.00 mmol) was dried and dissolved in
dry toluene (12.5 mL) in a flame-dried glass ampoule sealed
with a three-way stop cock. The monomer solution was
cooled to 0 ꢁC, and the PMP-FlLi solution (1.01 mL, 0.101
mmoL) was added with a syringe to initiate the polymeriza-
tion. After 24 h of initiation, the reaction was terminated by
adding MeOH (0.5 mL). The reaction mixture was poured
into a large excess of MeOH (200 mL), and the MeOH-insolu-
ble part was collected with a centrifuge and dried under
high vacuum at 60 ꢁC for 2 h. The MeOH-insoluble product
was obtained as colorless powder (0.448 g, yield 89.5%).
ꢁ
The reaction was continued at 60 C for additional 6 h (con-
1
version >99% by H NMR analysis). The solvents were then
removed using a rotary evaporator, and the residue was frac-
tionated into MeOH-insoluble and -soluble parts. The MeOH-
soluble part was suspended in 0.2 mL of CHCl3, and CH2N2
in ether was added to methylate the sample. The methylated
product was subjected to preparative SEC separation to
remove side-chain residues from the obtained poly(methyl
acrylate) (0.0079 g, 94%).
X-Ray Crystal Structure Analysis of BBPMFA
Crystal data were collected on a Bruker SMART Apex II CCD
diffractometer with graphite-monochromated Mo Ka radia-
tion (k ¼ 0.71073 Å) at 90 K. The crystal structure was
solved by direct methods (SHELXS-97)9 and refined by full-
matrix least-squares methods on F2 (SHELXL-97)10 with
APEX II software. The carbon and oxygen atoms were
refined anisotropically. All hydrogen atoms were located at
calculated positions and refined isotropically. Crystal data for
BBPMFA: C37H38O2; M ¼ 514.67; 0.63 ꢂ 0.38 ꢂ 0.35 mm3;
monoclinic; space group P21(no. 4); a ¼ 14.516(3) Å, b ¼
6.3789(15) Å, c ¼ 17.064(4) Å, a ¼ 90ꢁ, b ¼ 112.662(3)ꢁ, c
¼ 90ꢁ; V ¼ 1458.0(6) Å3; Z ¼ 2, qcalcd ¼ 1.172 g cmꢀ3; l ¼
0.071 mmꢀ1; T ¼ 90 K; 2hmax ¼ 53.46ꢁ; reflections col-
lected: 7831, independent reflections: 3367 (Rint ¼ 0.0573),
R1(I > 2r) ¼ 0.0508, wR2(I > 2r) ¼ 0.1282; final difference
map within þ0.279 and ꢀ0.293 eÅ3. CCDC 790370 contains
the supplementary crystallographic data for this article.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Conversion of Poly(BBPMFA) into poly(methyl acrylate)
Poly(BBPMFA) (Mn ¼ 10,200, Mw/Mn ¼ 3.04, run 8 in Table
1) (0.050 g, 0.097 mmol (monomeric unit)) was dissolved in
a mixture of CDCl3 (4.0 mL) and MeOH (0.50 mL) in a 20-
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
947

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of BBPMFA.
Photo Irradiation Experiments
BBPFA, the parent compound,7 in crystal is in the dihedral
angel around the C(9-position of fluorene ring)-O(ester)
bonding. While the angle was ca 10ꢁ in the BBPFA struc-
ture,7 it was nearly 180ꢁ in the BBPMFA structure. The com-
plete flip of the acryloyl group in BBPMFA in comparison to
that in BBPFA would be reasonably ascribed to steric repul-
sion between the carbonyl oxygen and the methyl group at
the 9-position of the fluorene ring. Thus, the additional
methyl group remarkably affects the monomer conformation.
Irradiation experiments were conducted using a Ushio Opti-
cal Modulex SX-UID500MAMQQ 500-W Hg-Xe lamp with or
without a Shimadzu SPG-120S monochromator.
Measurements
The 1H NMR spectra were recorded on a JEOL JNM-ECP400
spectrometer (400 MHz for 1H measurement) and a JEOL
JNM-EX270 spectrometer (270 MHz for 1H measurement).
SEC was carried out using a chromatographic system consist-
ing of a Hitachi L-7100 pump, an L-7420 UV detector (254
nm), and an L-7490 RI detector equipped with TOSOH TSK
gel G3000HHR and G6000HHR columns connected in series
(eluent, THF; flow rate, 1.0 mL/min). Preparative SEC was
performed using a JAI LC-908 preparative recycle chromato-
graph equipped with JAIGEL-1H and JAIGEL-2H columns con-
nected in series using CHCl3 as eluent. IR spectra were
measured with a Thermo-Fischer Nexus 870 spectrophoto-
meter using KBr pellet samples. Absorption spectra were
measured with JASCO V-550 and V-570 spectrophotometers.
Circular dichroism (CD) spectra were taken on a JASCO J-820
spectrometer. Specific rotation was measured using a JASCO
P-1030 digital polarimeter. MALDI-Mass spectra were
obtained with a Bruker Microflex spectrometer using dithra-
nol and silver trifluoroacetate as matrices.
Polymerization of BBPMFA
The conditions and results of free-radical polymerization
using AIBN or n-Bu3B-air and asymmetric anionic polymer-
ization (helix-sense-selective polymerization) using PMP-FlLi
are summarized in Table 1. The free-radical polymerizations
gave polymers having relatively low Mns’ at low monomer
ꢁ
ꢁ
conversions both at 60 C and ꢀ30 C (run 1 and 2). This is
in sharp contrast to the fact that BBPFA afforded a polymer
with Mn ¼ 6000 at 85% conversion (65% MeOH-insoluble
yield) by free radical polymerization at 60 ꢁC with AIBN in
RESULTS AND DISCUSSION
Synthesis and Structure of BBPMFA Monomer
BBPMFA was synthesized by a four-step reaction according
to Scheme 1 starting from 2,7-dibromofluorene and was
purified by recrystallization to give a material pure enough
to be used for anionic polymerization. The monomer struc-
ture was confirmed by single-crystal X-ray crystallography
(Fig. 1). The most obvious difference between BBPMFA and
FIGURE 1 X-Ray crystal structure of BBPMFA monomer.
948
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
addition, these polymers showed intense CD spectra (Fig. 2)
whose intensity increased with an increase in Mn. These
results strongly suggest that the observed chiroptical proper-
ties of poly(BBPMFA)s obtained using PMP-FlLi arise from
preferred-handed helical conformation. Although the CD
spectral pattern of poly(BBPMFA) was very similar to that
of poly(BBPFA), g value for poly(BBPMFA) (3.75 ꢂ 10ꢀ4 at
344 nm (Mn 5400)) was greater than that for poly(BBPFA)
(2.02 ꢂ 10ꢀ4 at 334 nm (Mn 12,730)).7 This may mean that
poly(BBPMFA) has a greater preferred handedness than poly
(BBPFA). The CD spectral intensities of the two polymers
may be directly compared because the two polymers have
the same side-chain chromophore.
For the purpose of tacticity analysis, the polymer from run 8
in Table 1 was converted to poly(methyl acrylate) by solvoly-
sis in a mixture of CHCl3 and MeOH followed by methylation
with CH2N2. Solvolysis of (meth)acrylic polymers having
bulky side groups can be performed using an acid such as
HCl and H2SO4. However, main-chain racemization has been
reported for the side-chain hydrolysis reaction of helical poly
(N,N-diphenylacrylamide) under acidic conditions.11 Hence,
in order to avoid possible main-chain racemization, solvoly-
sis of poly(BBPMFA) was carried out under a neutral condi-
tion. Figure 3 shows the 1H NMR spectrum of poly(methyl
acrylate) obtained from poly(BBPMFA). From the ratio of
the peak areas of main-chain methylene signals due to meso
and racemo diads, diad tacticity was estimated to be m/r ¼
57/43. This indicates that poly(BBPMFA) obtained by ani-
onic polymerization is only slightly isotactic. This result is in
a sharp contrast to the fact that most helical (meth)acrylic
polymers so far reported are highly isotactic. In the polymer-
ization of BBPMFA using PMP-FlLi, only conformational con-
trol may be achieved unlike helix-sense-selective polymeriza-
tions of other (meth)acrylic monomers including that of
triphenylmethyl methacrylate where both conformation and
configuration are simultaneously, perfectly controlled.4
FIGURE 2 CD (top panel) and UV (bottom panel) spectra of
poly(BBPMFA)s having different degrees of polymerization: Mn
10,200 (run 8 in Table 1) (A), Mn 2800 (run 6 in Table 1) (B),
and Mn 2,800 (run 6 in Table 1) (C). [conc. 2.00 ꢂ 10ꢀ4 M per
monomeric residue, in CHCl3, at room temperature]. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
CHCl3 ([M]o ¼ 0.11 M, [AIBN]o ¼ 2.2 mM, time ¼ 12 h).7 An
increase in bulkiness of BBPMFA by the size of a methyl
group may seriously retard propagation in the radical
polymerization.
ꢁ
Asymmetric anionic polymerization with PMP-FlLi at ꢀ78 C
led to optically active polymers with Mns’ in the range of ca.
2000–3000 although the monomer conversion ratios were
not quantitative at all [M]/[I] ratios examined (runs 3–5).
Under similar conditions with the same initiator system,
BBPFA quantitatively led to polymers with Mns’ up to 8000.7
This again probably means that BBPMFA isꢁ too bulky to be
polymerized as smoothly as BBPFA at ꢀ78 C in the anionic
polymerization. Hence, the introduction of single methyl
group to the side chain resulted in a large decrease in reac-
tivity of BBPMFA monomer in both free-radical and anionic
modes.
Structural Transition of Helical Poly(BBPMFA) Through
the Interaction with Light (Photolysis)
However, in the anionic polymerization, monomer conver-
sions were remarkably improved by raising the reaction tem-
perature to 0 ꢁC (runs 6–9). At [M]/[I] ¼ 5 and 10, the
monomer was quantitatively consumed in 24 h at 0 ꢁC. In
the range of [M]/[I] ¼ 5–15, Mn of the obtained polymers
increased with [M]/[I] up to Mn ¼ 10,200 (run 8). At [M]/[I]
¼ 20, most part of the obtained material which probably is
of higher molecular weight was insoluble in THF or CHCl3,
and Mn of the soluble part was quite low (1300). In addition,
the reaction systems at [M]/[I] ¼ 15 and 20 gelled in the
course of polymerization; this may be the main reason why
the monomer was not quantitatively polymerized in these
two systems.
As mentioned earlier, preferred-handed helical, optically
active poly(BBPFA) is readily transformed into racemic form
through helix-helix transition induced by light although the
The polymers obtained atꢁ0 ꢁC were optically active as well
as those obtained at ꢀ78 C. The ones obtained at [M]/[I] ¼
10 and 15 with higher Mns’ (Mn ¼ 5400 and 10,200, respec-
tively) showed much larger dextrorotation than the one
obtained at [M]/[I] ¼ 5 with a lower Mn (Mn ¼ 2800). In
FIGURE 3 1H NMR spectrum of poly(methyl acrylate) derived
from the poly(BBPMFA) (Mn ¼ 10,200, run 8 in Table 1). ꢂ
denotes impurities. [400 MHz, CDCl3, 60 ꢁC].
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
949

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ꢂ 10ꢀ2 M per monomeric residue, 1-cm quartz cell) by irradi-
ation with a 500-W Hg-Xe lamp and was monitored by 1H
NMR spectroscopy [Fig. 6(A)]. A similar experiment was also
performed for BBPMFA monomer [Fig. 6(B)]. The polymer
showed broad, rather structureless peaks in the aliphatic and
aromatic regions [Fig. 6(A-1)]. The broad peaks can be
ascribed to a rigid, helical polymer structure that results in a
1
relatively short relaxation time in the H NMR experiment. Af-
ter irradiation for 5 min, some sharp peaks that seem to be
based on low-molecular-weight compounds emerged and the
originally broad, aliphatic and aromatic peaks became nar-
rower [Fig. 6(A-2)]. The most characteristic in the spectrum
after irradiation was the sharp, singlet signal at 6.17 ppm in
the olefinic proton signals region for vinyl monomers. This sig-
nal and most of the sharp peaks in the aromatic region coin-
cided in chemical shift with those of 2,7-bis(4-t-butylphenyl)-
9-methylenefluorene (2,7-bis(4-t-butylphenyl)dibenzofulvene)
[Fig. 6(C)]. The authentic sample of this olefin compound was
synthesized from 2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-
ol, the source compound for the BBPMFA monomer synthesis.
In addition, MALDI-mass spectra of the irradiated sample indi-
cated a clear signal at m/e ¼ 442 corresponding to the molec-
ular weight of the olefin compound. Therefore, it is assumed
that irradiation of the polymer causes photolysis of the ester
linkage giving the olefin.
FIGURE 4 Change in CD (top panel) and UV (bottom panel)
spectra of the poly(BBPMFA) (Mn ¼ 5400, run 7 in Table 1)
upon irradiation to the sample in a CHCl3 solution in a 1-mm
quartz cell using a 500-W Hg-Xe lamp without monochromat-
ing for 0 s (A), 0.5 s (B), and 1 s (C). [conc. 2.00 ꢂ 10ꢀ4 M per
monomeric residue, at room temperature]. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
polymer helix is stable on heating.7 Because a similar confor-
mational transition would be plausible for poly (BBPMFA)
synthesized in this work, its conformational and structural
stability was examined.
The lowest-molecular-weight peak observed in the SEC curve
after irradiation shown in Figure 5 is based on this olefin.
This was confirmed by analyzing the authentic olefin by SEC
as a reference compound. Although the compounds showing
other low-molecular-weight peaks in SEC have not yet been
identified, they could contain photo reaction products
obtained from the olefin.12,13
Prior to the photo-induced structural studies, stability of
poly(BBPMFA) toward thermal stimulus was tested. Struc-
tural changes were monitored by CD and UV spectra. A
CHCl3 solution of the poly(BBPMFA) (Mn ¼ 5400, run 7 in
Table 1) (conc. 2.00 ꢂ 10ꢀ4 M per monomeric residue) was
heated at 60 ꢁC for 24 h. Before and after heating, no clear
change was observed in either CD or UV, indicating that the
helical conformation and its preferred handedness were
unaffected by heat under the conditions in this work.
Also in the spectrum of the irradiated BBPMFA monomer
sample in CDCl3, the vinyl signal of the olefinic compound is
However, upon irradiation to a CHCl3 solution of poly
(BBPMFA) (Mn ¼ 5400, run 7 in Table 1) (conc. 2.00 ꢂ
10ꢀ4 M per monomeric residue, 1-mm quartz cell) using a
500-W Hg-Xe lamp, CD absorption bands completely disap-
peared within 1 s (Fig. 4). At the same time, the UV spectral
pattern largely altered, indicating that chemical transforma-
tion had taken place to the polymer. In the case of photo-
induced helix-helix transition of poly(BBPFA), no clear
change was found in UV spectra on irradiation.
The chemical transformation was evidenced by SEC, 1H
NMR, and IR analyses (Figs. 5, 6, and 7, respectively). The
peak position in the SEC chromatogram shifted to lower-mo-
lecular-weight ranges (Fig. 5). This indicates that a rigid,
helical polymer chain transformed into a compact shape or
that lower-molecular-weight compounds were formed from
the original polymer through photochemical reactions. The
latter would be more plausible based on the remarkable
change in the UV spectrum of the polymer.
FIGURE 5 SEC curves of poly(BBPMFA) (Mn ¼ 5400, run 7 in
Table 1) (A) and the products obtained from the polymer upon
irradiation using a 500-W Hg-Xe lamp without monochromat-
ing for 1 s (B). The samples were obtained from the irradiation
experiment for Figure 4. SEC curves were obtained by UV
detection at 254 nm. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
To obtain information on the change in chemical structure of
the polymer, the reaction was conducted in CDCl3 (conc. 2.00
950
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 6 1H NMR spectra of poly(BBPMFA) (Mn ¼ 5400, run 7 in Table 1) (A-1) and the products obtained from the polymer upon irra-
diation for 5 min (A-2), those of BBPMFA monomer (B-1 and B-10) and the products obtained from the monomer upon irradiation for 5
min (B-2 and B-20), and that of 2,7-bis(4-t-butylphenyl)-9-methylenefluorene (2,7-bis(4-t-butylphenyl)dibenzofulvene) (C). ꢂ denotes
impurities. Irradiation experiments were performed in CDCl3 at 2.00 ꢂ 10ꢀ2 M concentration (per monomeric residue) in a 1-cm quartz
cell for the polymer and the monomer using a 500-W Hg-Xe lamp without monochromating. [270 MHz, CDCl3, room temperature].
clearly seen [Fig. 6(B-20)]. The other signals in the olefinic
proton signals region after irradiation to the monomer are
assigned to those of BBPMFA monomer and those of acrylic
acid. These results confirm that irradiation causes a photo-
lytic ester linkage cleavage leading the olefin and acid. Based
on these results, the products of the polymer photolysis are
reasonably identified as a mixture of the olefin and poly
(acrylic acid) (Scheme 2). Photolysis of 9-methyl- or 9-alkyl-
fluorene derivatives leading to olefin compounds has been
reported.12,13 The intermediates of such reactions have been
proposed to be a fluorenyl cation.
It should be noted here that the polymer photolysis reaction
shown in Figure 6(A) seems much slower than the spectral
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
951

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
tra. This is because 2,7-bis(4-t-butylphenyl)-9-methylene-
fluorene and 2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl
ester show distinguishable spectral patterns. Figure 8 shows
the UV spectra of poly(BBPMFA), BBPMFA monomer and
corresponding photolysis reaction mixtures obtained by irra-
diation at 325 nm for various periods of time and that of 2,
7-bis(4-t-butylphenyl)-9-methylenefluorene. 2,7-Bis(4-t-butyl-
phenyl)-9-methylenefluorene shows characteristic absorption
bands in the range of 260–290 nm with the peak maximum
at 280 nm. In this range, no clear, overlapping peaks due
to 2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl ester are
observed.
The photolysis reactions described in this section was inten-
tionally set to be slower than that in Figure 4 to increase ac-
curacy in time axis using the monochromated incident light.
Monochromating at 325 nm leads to a lower number and a
lower averaged-energy of photons that hit the sample. The
lower averaged energy of photons comes from the absence
of the UV-range light including the Hg 254 nm line.
FIGURE 7 FT-IR spectra of poly(BBPMFA) (Mn ¼ 5400, run 7 in
Table 1) and the products obtained from the polymer upon
irradiation using a 500-W Hg-Xe lamp without monochromat-
ing for 5 min. The samples were obtained from the irradiation
experiment for Figure 6A. [KBr pellet].
The photolysis mixtures obtained from poly(BBPMFA) and
BBPMFA showed an isosbestic points at 295 nm at earlier
stages of photolysis, indicating that photolysis systems con-
sist only of original 2,7-bis(4-t-butylphenyl)-9-methylfluoren-
9-yl ester and resulting 2.7-bis(4-t-butylphenyl)-9-methylene-
fluorene as chromophores at earlier stages of the reaction.
Based on these spectral characteristics, the extent of photoly-
sis was estimated. By monitoring the increment in molar
absorptivity at 280 nm with reference to molar absorptivity
at 295 nm in the course of irradiation, photolysis conversion
versus irradiation time plots were obtained for poly(-
BBPMFA) samples having different Mns’ (Mn 5400, 10,200,
22,800) and BBPMFA monomer (Fig. 9). The polymer sam-
ple of Mn ¼ 22,800 was obtained by SEC fractionation from
the sample of Mn ¼ 10,200 (run 8 in Table 1).
change shown in Figure 4. This may be partially because a
higher-concentration sample undergoes slower transforma-
tion possibly due to intermolecular interaction and partially
because a sufficient number of photons was not supplied for
the sample due to the existence of too many absorbing mole-
cules at the concentration of 2.00 ꢂ 10ꢀ2 M per residue.
The production of poly(acrylic acid) on photolysis of the
polymer was supported by the IR spectrum of the photolysis
products indicating the AOH signal around 3500 cmꢀ1 and
the broadened carbonyl stretching signals shifted to the
lower-wave number direction (Fig. 7). The products may be
a mixture of unreacted ester, acid, and 2,7-bis(4-t-butyl-
phenyl)-9-methylenefluorene.
Kinetic Aspects of Photolysis
Photolysis of BBPMFA monomer was faster than that of
poly(BBPMFA) samples, and polymers having different Mns’
Photolysis of poly(BBPMFA) and BBPMFA monomer can
be monitored by UV spectra as well as H NMR and IR spec-
1
SCHEME 2 Photolysis of BBPMFA monomer (top) and poly(BBPMFA) (bottom).
952
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
underwent photolysis at very similar rates. This is consistent
with that polymer reactions are generally slower than corre-
sponding reactions involving low-molecular-weight com-
pounds.14 First-order rate constants (k) of photolysis for
BBPMFA monomer, poly(BBPMFA) of Mn ¼ 5400, poly
(BBPMFA) of Mn ¼ 10,200, and poly(BBPMFA) of Mn
¼
22,800 were 0.028 sꢀ1, 0.011 sꢀ1, 0.011 sꢀ1, and 0.012 sꢀ1
,
respectively, as estimated from the data shown in Figure 9
according to:
lnð½esterꢃo=½esterꢃtÞ ¼ kt
where [ester]o, [ester]t, k, and t are the initial concentration
of 2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl ester, con-
centration of the ester at given reaction time, first-order rate
constant, and reaction time, respectively. In this equation,
the concentration of photons involved in photolysis is not
taken into account. Hence, the rate constant values estimated
here are not intrinsic to these particular monomer and poly-
mer compounds and will vary depending on light intensity.
However, it is interesting to compare the rate constants for
photolysis with those for chemical solvolysis of the side-
chain ester linkage in the dark. The latter values were esti-
mated to be 0.018 hꢀ1 for the reaction system under a neu-
tral condition (polymer (Mn ¼ 5400) 10 mg, CDCl3 0.8 mL,
MeOH 0.1 mL, room temperature) and 0.17 hꢀ1 for the reac-
tion system under an acidic condition (polymer (Mn ¼ 5400)
10 mg, CDCl3 0.8 mL, MeOH 0.1 mL, conc. HCl 5 lL, room
1
temperature) by monitoring the reactions by H NMR. Thus,
the photolysis is 250–2400 times faster than the solvolysis.
Furthermore, it is faster than reported solvolysis of other
helical polymethacrylates under neutral conditions14 by the
order of 102. Although rate constants have not been reported
for most polymer reactions,15 solvolysis of helical polymetha-
crylates is considered to be a fast class of macromolecular
reaction. It has been reported that conversion in hydrolysis
reaction of poly(methyl methacrylate) in a mixture of MeOH
FIGURE 8 UV spectra of poly(BBPMFA) (Mn 5400, run 7 in Ta-
ble 1) (A) and BBPFMA monomer (B) upon irradiation to sam-
ple solutions in CHCl3 in a 1-mm quartz cell at 325 nm for
different periods of time and that of 2,7-bis(4-t-butylphenyl)-9-
methylenefluorene (2,7-bis(4-t-butylphenyl)dibenzofulvene) in
CHCl3 (C). [conc. 2.00 ꢂ 10ꢀ4 M per monomeric residue, at
room temperature]. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 9 First-order plots for photolysis of BBPMFA monomer
(A) and poly(BBPMFA) samples having different Mns’ (Mn 5400
(B), 10,200 (C), 22,800 (D)). The reactions were carried out in a
CHCl3 solution at a concentration of 2.00 ꢂ 10ꢀ4 M per mono-
meric residue in a 1-mm quartz cell with irradiation at 325 nm.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
953

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 3 Photolysis of a helical polymer by unzipping (A)
and random (B) mechanisms. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
and water containing NaOH at 80 ꢁC is less than 1% in
3 h.16
Because CD and UV spectral changes were complete within 1
s when the irradiation was conducted using a 500-W Hg-Xe
lamp without monochromating (Fig. 4), the photolysis of hel-
ical poly(BBPMFA) may be one of the fastest macromolecu-
lar reactions leading to a large change in molecular shape.14
For side-chain solvolysis of helical poly(meth)acrylates,
‘‘unzipping’’ mechanism17 has been proposed. According to
this mechanism, once an ester bonding is cleaved in a chain
(the initial cleavage), typically at the chain terminals, solvoly-
sis along the entire chain proceeds from the initial cleavage
point at a much large rate than that of the initial cleavage.
This mechanism involving acceleration can be explained by
the acidic catalytic effect of the carboxylic acid moiety
formed by solvolysis for the neighboring monomeric units. A
reaction system in the course of side-chain cleavage reaction
through the unzipping mechanism should be, ideally, a mix-
ture of intact helix and completely decomposed products
without partially decomposed chains17 [Scheme 3(A)] while
a system with random scission mechanism may contain
chains with various degrees of decomposition [Scheme 3(B)].
Figure 10 shows the SEC curves of the irradiated products
obtained from the poly(BBPMFA) of Mn ¼ 5400 at different
conversions. Up to 53% photolysis conversion, the position
and shape of polymer peaks virtually did not change, the rel-
ative intensity of polymer peaks decreased, and the relative
intensity of lower-molecular-weight peaks increased. These
results indicate that the reaction mixture does not contain
partially decomposed chains, establishing the unzipping pho-
tolysis mechanism.
FIGURE 10 SEC curves of poly(BBPMFA) (Mn ¼ 5400, run 7 in
Table 1) corresponding to the spectral experiments shown in
Figure 8A: irradiation time 0 min (conv. 0%) (A), 10 s (conv.
11%) (B), 1 min (conv. 53%) (C), and 10 min (conv. 80%) (D).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
course photolysis (Fig. 11). CD intensity decreased with an
increase in photolysis conversion. To examine the change in
CD intensity quantitatively, molar ellipticity was plotted
against photolysis conversion as shown in Figure 12. In
When the unzipping mechanism is in effect, the rate-limiting
step is the initial cleavage. Hence, the rate constants of pho-
tolysis discussed above are actually those for the initial
cleavage reaction. Rate constants for the following photolysis
should be much larger, i.e., the following photolysis should
be much faster although estimating them is beyond the
scope of this work.
FIGURE 11 CD (top panel) and UV (bottom panel) spectra of
poly(BBPMFA) (Mn ¼ 5400, run 7 in Table 1) corresponding to
the spectral experiments shown in Figure 8A. The UV spectra
are identical to those shown in Figure 8A [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Helix-Helix Transition (Racemization) versus Photolysis
Does poly(BBPMFA) racemize before it decomposes? This
question was addressed by CD spectral measurements in the
954
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
Figure 12, the solid line indicates the theoretical relation
between the two quantities expected for a system where the
decrease in CD intensity is based only by decomposition. If
CD intensity is lower than the theoretical line, the decrease
is not only on decomposition but also by racemization.
In Figure 12, all data points are below the line, indicating
that racemization did take place. Racemization precedes pho-
tolysis for a chain although the chain may start to decom-
pose before completion of racemization.
The original intention of this work was to design a new
polymer that undergoes photo-induced helix-helix transition
and to examine how the methyl group introduced at the 9-
position of fluorene ring affects the stereomutation. Because
the polymer decomposes on irradiation as well as racemiza-
tion, it is difficult to examine and discuss racemization only.
However, by irradiating the polymer of Mn ¼ 5400 at 254
nm, information on the effects of the methyl group on race-
mization was obtained (Fig. 13). After irradiation at 254 nm
for 1 min, the CD intensity decreased to 43% of the original
spectrum at 343 nm and the photolysis conversion was only
5%. This means that the large decrease in CD intensity is
mainly due to helix-helix transition (racemization). This is in
a sharp contrast to the reported result that poly(BBPFA)
without an additional methyl group did not show CD spec-
tral changes on irradiation at 254 nm. From these results, it
can be concluded that the added methyl group at the 9-posi-
tion of fluorene ring largely facilitates the stereomutation.
FIGURE 13 CD (top panel) and absorption (bottom panel) spec-
tra of poly(BBPMFA) (Mn ¼ 5400, run 7 in Table 1) upon irradi-
ation to sample solutions in CHCl3 in a 1-mm quartz cell at 254
nm for different periods of time. [conc. 2.00 ꢂ 10ꢀ4 M per
monomeric residue, at room temperature]. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
its chirality. The photolysis reaction found in this work may
be one of the fastest among polymer reactions most of which
are caused mainly by chemical stimuli.
The reason why the additional methyl group at the 9-posi-
tion of the fluorene ring of BBPMFA and poly(BBPMFA)
makes the monomer so unstable on irradiation is not imme-
diately clear. A possible explanation may be that production
of 2,7-bis(4-t-butylphenyl)dibenzofulvene is energetically
very favorable.
CONCLUSIONS
Preferred-handed helical, optically active poly(BBPMFA) was
synthesized by helix-sense-selective anionic polymerization
of BBPMFA monomer. Both the monomer and the polymer
were found to readily decompose on irradiation. The appa-
rent, first-order rate constants of polymer photolysis on irra-
diation at 325 nm were in the order of 10ꢀ2 sꢀ1 while that
for solvolysis in the dark under a neutral condition was as
high as 0.018 hꢀ1. Thus, the photolysis confirmed in this
work is far much faster than solvolysis. Within 1 s of irradia-
tion with a 500-W Xe-Hg lamp, the polymer completely lost
Photolysis of 9-methyl- and 9-alkylfluorene derivatives simi-
lar to that found in this work has been reported; however,
such reactions have never been used for photo-induced
structural transformation of a polymer with an ordered con-
formation. 2,7-Bis(4-t-butylphenyl)-9-methylfluorenyl esters
(polymer and monomer) which were first made in this work
and were found to be effectively and quickly transformed
into an olefin compound and an acid may be applicable for
other systems that need fast modification of properties of
polymers by remote, photo stimuli. An example is helical
template for molecular-imprinting synthesis of chiral cross-
linked polymer (gel) where the template needs to be readily
degradable.18
In addition, photolysis of racemic poly(BBPMFA) prepared
under achiral conditions using circularly polarized light
could possibly be helix-sense-selective. This might be a novel
way to obtain a single-handed helical polymer. Further stud-
ies on this possibility are under way.
This study was supported in part by the MEXT through Grants-
in-Aid No. 21655037, No. 22350047, and No. 20111009 (Scien-
tific Research on Innovative Areas ‘‘Emergence in Chemistry’’)
and through the Global COE Program (Project No. B01: Cataly-
sis as the Basis for Innovation in Materials Science) and in part
by the Asahi Glass Foundation.
FIGURE 12 CD intensity at 344 nm versus photolysis conver-
sion plots corresponding to the data shown in Figure 11. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
A PHOTO-DEGRADABLE HELIX, SAKAMOTO ET AL.
955

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
7 Sakamoto, T.; Fukuda, Y.; Sato, S.-I.; Nakano, T. Angew Chem
REFERENCES AND NOTES
Int Ed Engl 2009, 48, 9308–9311.
1 (a) Okamoto, Y.; Nakano, T. Chem Rev 1994, 94, 349–372; (b)
Nakano, T.; Okamoto, Y. Chem Rev 2001, 101, 4013–4038; (c) Oka-
moto, Y.; Yashima, E. Prog Polym Sci 1990, 15, 263; (d) Okamoto,
Y.; Nakano, T.; Habaue, S.; Shiohara, K.; Maeda, K. J Macromol
Sci Pure Appl Chem 1997, A34, 1771; (e) Nakano, T.; Okamoto, Y.
Macromol Rapid Commun 2000, 21, 603–612; (f) Okamoto, Y.;
Nakano, T. Catalytic Asymmetric Synthesis, 2nd ed.; Wiley: New
York, 2000; pp 757–796; (g) Smaldone, R. A.; Moore, J. S. Chem
Eur J 2008, 14, 2650–2657; (h) Hill, D. J.; Mio, M. J.; Prince, R. B.;
Hughes, T. S.; Moore, J. S. Chem Rev 2001, 101, 3893–4011; (i)
Green, M. M.; Cheon, K.-S.; Yang, S.-Y.; Park, J.-W.; Swansburg,
S.; Liu, W. Acc Chem Res 2001, 34, 672–680; (j) Yashima, E.;
Maeda, K. Macromolecules 2008, 41, 3–12; (k) Yashima, E.;
Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem Rev 2009, 109,
6102–6211; (l) Yashima, E.; Maeda, K.; Furusho, Y. Acc Chem Res
2008, 41, 1166–1180; (m) Kumaki, J.; Sakurai, S.; Yashima, E.
Chem Soc Rev 2009, 38, 737–746.
8 Yamada, K.; Nakano, T.; Okamoto, Y. Macromolecules 1998,
31, 7598–7605.
9 Sheldrick, G. M. SHELXS-97, A program for crystal structure
determination; University of Gottingen, Gottingen, Germany,
¨
¨
1997.
10 Sheldrick, G. M. SHELXL-97, A program for crystal structure
refinement; University of Gottingen, Gottingen, Germany,
¨
¨
1997.
11 Shiohara, K.; Habaue, S.; Okamoto, Y. Polym J 1996, 28,
682–685.
12 Mladenova, G.; Singh, G.; Acton, A.; Chen, L.; Rinco, O.;
Johnston, L. J.; Lee-Ruff, E. J Org Chem 2004, 69, 2017–2023.
13 Blazek, A.; Pungente, M.; Krogh, E.; Wan, P. J Photochem
Photobiol A 1992, 64, 315–327.
14 Sakamoto, T.; Nishikawa, T.; Fukuda, Y.; Sato, S.-I.; Nakano,
2 Okamoto, Y.; Suzuki, K.; Ohta, K.; Hatada, K.; Yuki, H. J Am
T. Macromolecules 2010, 43, 5956–5963.
Chem Soc 1979, 101, 4763–4765.
15 Stevens, M. P. Polymer Chemistry; Oxford University Press:
3 (a) Okamoto, Y.; Suzuki, K.; Yuki, H. J Polym Sci Part A:
Polym Chem 1980, 18, 3043–3051; (b) Okamoto, Y.; Shohi, H.;
Yuki, H. J Polym Sci Part C: Polym Lett 1983, 21, 601–607.
New York, 1999; pp 259–282.
16 Sandner, B.; Bischof, C. Angew Macromol Chem 1983, 115,
207–219.
4 Nakano, T.; Okamoto, Y.; Hatada, K. J Am Chem Soc 1992,
114, 1318–1329.
17 Hosoda, M.; Schonhausen, U.; Pino, P. Makromol Chem 1993,
¨
194, 223–232.
5 Nakano, T. J Chromatogr A 2001, 906, 205–225.
6 Okamoto, Y.; Mohri, H.; Nakano, T.; Hatada, K. J Am Chem
18 Nakano, T.; Satoh, Y.; Okamoto, Y. Macromolecules 2001,
Soc 1989, 111, 5952–5954.
34, 2405–2407.
956
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA